It is well known that an aqueous solution can corrode an untreated metal. Therefore, metal articles, such as metal containers for water-based products, must be rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible, and more adhesive coating, but often, improving one advantageous property is achieved at the expense of a second advantageous property.
In addition, practical considerations limit the thickness, adhesive properties and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal container must be able to withstand the solvating properties of a product packaged in the metal container. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, such as beer, by imparting an off-taste to the product.
Conventionally, organic solvent-based coating compositions were used to provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container. However, because of perceived environmental and toxicological concerns, and in order to comply with increasingly strict governmental regulations, an increasing number of coating compositions are water-based. Unfortunately, many water-based coating compositions include ingredients that are water-soluble or water dispersible, and, therefore, cured coatings resulting from water-based coating compositions often are more susceptible to the solvating properties of water. In addition, water-based coating compositions do not completely overcome the perceived environmental and toxicological problems associated with organic solvents because water-based compositions often contain some organic solvent.
Therefore, in order to entirely avoid the perceived environmental and toxicological problems associated with organic solvents, investigators have sought solid coating compositions that can be applied to a metal substrate. To date, investigators have had difficulty in providing a solid coating composition that compares favorably to a liquid coating composition with respect to film uniformity, film appearance, and film performance.
In prior attempts to find a useful solid coating composition, investigators have tested powder coatings, laminated film coatings, radiation cure coatings, and extrusion coatings. Attempts have been made to use free film laminates of polymers such as polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE). In this method, a preformed polymer film, about 10 to about 25 microns thick, is applied to the metal substrate. The film laminate method is a rapid method of coating a metal substrate, but the method is generally either too expensive and/or the coated metal substrate does not possess all of the required properties. Solid powder coatings also have been used to coat a metal substrate with a coating composition. However, the application of a thin, uniform coating to a metal substrate is difficult using the powder coating method. Solid coating compositions also have been extruded onto a metal substrate. These attempts have encountered serious technical or economical problems.
Investigators, therefore, have long sought a solid coating composition for use on the exterior and interior of food and beverage containers that exhibits the advantageous properties of adhesion, flexibility, chemical resistance, and corrosion inhibition, and that is economical and does not adversely affect the taste or other esthetic properties of sensitive foods and beverages packaged in the container. Investigators especially have sought useful solid coating compositions in order to reduce the perceived environmental and toxicological concerns associated with organic solvents. In particular, investigators have sought a solid coating composition for food and beverage containers that meets increasingly strict environmental regulations, has corrosion inhibition properties comparable to existing organic solvent-based coating compositions, and is easily applied onto a metal substrate as a thin, uniform film. Such a coating composition would satisfy a long felt need in the art.
For convenience, the following description will be mainly directed to metal packaging articles but it will be understood by those skilled in the art that other articles (e.g., caps and closures) may be fabricated using the films and methods of the invention.